Fault current limiter

ABSTRACT

A fault current limiter (FCL) for limiting a fault current in a power line during a fault condition. The FCL includes a magnetic coupling circuit for monitoring current in the power line through magnetic coupling; a sensing circuit for sensing the current in the power line and providing a signal indicative of the sensed current; a control circuit receiving the signal indicative of the sensed current in the power line and determining whether the sensed current indicates that the fault condition exists; and high and low impedance paths that are connected in parallel. The high impedance path includes a discharging impedance circuit for limiting the fault current. The low impedance path includes a reactor circuit and a switching unit having an ON state for conducting current through the low impedance path and an OFF state for conducting current through the high impedance path.

FIELD OF THE INVENTION

The present invention relates generally to fault current limiters, andmore particularly to a non-superconducting fault current limiter.

BACKGROUND OF THE INVENTION

As the demand for electric power increases, new power sources (such asIndependent Power Producers (IPPs) and Distributed Generators (DGs)) arebeing added to the power grid. The consequence of this trend is that thefault current in the power system is being increased, exceeding theratings of existing protection devices, e.g., circuit breakers, relaysand fuses.

The IPPs, installed in parallel with existing generators, decrease theequivalent source impedance of the grid. Furthermore, the DGs areusually placed close to the load and the fault. Therefore, the impedancefrom the DGs to fault is also decreased. The effect of smaller impedancein the network is that the fault current might rise above the ratedlimits of the existing protection devices. In order to protect equipment(e.g., transformers and electric machines) it becomes necessary toeither upgrade the protection devices or lower the threshold faultcurrent level. However, device upgrade is generally not economicallyviable, considering the enormous number of devices that needreplacement. Accordingly, fault current limiters (FCLs) have become apreferred option to address the over-rating issue.

Fault current limiter technology offers several advantages, including,but not limited to, the following: (i) mitigating the effect of highthreshold fault current levels on a distribution system, therebypermitting use of lower rated protection devices and deferring costlydevice replacements; (ii) protecting existing devices from the firstlarge peak during a fault condition, since many FCLs can limit the faultcurrent within the first quarter-cycle; (iii) reducing voltage dips; and(iv) enhancing grid stability.

Traditionally, FCLs have been applied in power generation, transmissionand distribution, and in all ranges of voltage levels from 400V to 132kV. Typical applications of FCLs include busbar coupling and transformerfeeder, distributed generation coupling, power plant auxiliaries, andship propulsion systems.

An FCL ideally possesses the following characteristics: (a) virtuallyzero impedance under normal operation; (b) fast detection and fastaction (e.g., detection of fault current within the first cycle); (c)minimum impact on existing protection relays and circuit breakers; (d)minimum impact on voltage magnitude and phase; and (e) automatic andfast recovery to address repeated faults.

Examples of existing FCL technologies include Superconductor FCL(SCFCL), Solid-State FCL (SSFCL), and Is-limiter. Each of these existingFCL technologies has drawbacks (e.g., high operation losses, bulky sizeand servicing/part replacement issues). Most SSFCLs and some SCFCLs aresubjected to switching losses during normal operation. Moreover, thesuperconductors in SCFCLs require extra energy to be cryogenized inorder to stay in the superconducting state during normal operation. ManyFCLs mentioned above are large in dimensions. They either need extracryogenic equipments, or need large capacitor banks or large iron coresto operate. An Is-limiter is comprised of an extremely fast switch,paralleled with a high rupturing capacity fuse. In order to achieve thedesired short opening time, the Is-limiter fires a pyrotechnic charge toopen the main conductor. The fault current is then carried by theparallel fuse, which interrupts the fault current at the next zerocrossing. The Is-limiter has several drawbacks. For example, theIs-limiter can raise safety concerns because it is not fail-safe, thatis, correct operation of the Is-limiter cannot be tested withoutdestroying the Is-limiter (i.e., the parallel fuse). Furthermore, supplyof pyrotechnic materials are regulated and constrained by the U.S.Department of Defense, thereby resulting in supply problems andincreased manufacturing costs. In addition, replacement of the parallelfuse is required after each triggering, thus leading to high operatingcost.

One existing Non-Superconducting FCL (NSCFCL) is a bridge-typenon-superconducting FCL proposed by M. T. Hagh and M. Abapour in“Nonsuperconducting Fault Current Limiter With Controlling theMagnitudes of Fault Currents;” Hagh, M. T. and Abapour M.; IEEETRANSACTIONS ON POWER ELECTRONICS; 2009; Vol. 24; No. 3; pp. 613-619,hereinafter referred to as “Hagh et al.” In Hagh et al.'s NSCFCL, a DCreactor serves two functions: (a) during normal operation, the DCreactor minimizes the current ripple of the rectified DC current; and(b) during a fault condition, the DC reactor is used as impedance duringthe initial rise of fault current. The rise of the fault current isslowed so that a control circuit has adequate time to operate asemiconductor switch. This NSCFCL has drawbacks that include, but arenot limited to: (1) the need for a semiconductor switching unit having ahigh voltage rating due to the high voltage stress; (2) the need for adischarging resistor having a large resistance; (3) the need forisolation transformers having high voltage ratings; and (4) large DCreactor size with current always through it in either the normal orfaulted state. These component requirements result in high manufacturingcosts for the NSCFCL.

The present invention provides a non-superconducting fault currentlimiter that overcomes these and other drawbacks of existing FCLs.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there isprovided a fault current limiter (FCL) for limiting a fault current in apower line during a fault condition. The FCL comprises: (a) a magneticcoupling circuit for monitoring current in the power line throughmagnetic coupling; (b) a sensing circuit for sensing the current in thepower line and providing a signal indicative of the sensed current; (c)a control circuit receiving the signal indicative of the sensed currentin the power line and determining whether the sensed current indicatesthat the fault condition exists; and (d) high and low impedance pathsconnected in parallel. The high impedance path includes a dischargingimpedance circuit for limiting the fault current, wherein thedischarging impedance circuit injects a discharging impedance into themain power line through magnetic coupling during the fault condition.The low impedance path includes (i) a reactor circuit; and (ii) aswitching unit having an ON state and an OFF state, wherein the state ofthe switching unit determines whether current conducts through the lowimpedance path or the high impedance path. The control circuitdetermines the state of the switching unit.

In accordance with another aspect of the present invention, there isprovided a power system comprising: (a) a power line that electricallyconnects a power source to a load; (b) a protection device forinterrupting a circuit to clear a fault condition; and (c) a faultcurrent limiter (FCL) for limiting a fault current in the power lineduring a fault condition. The FCL comprises: (i) a magnetic couplingcircuit for monitoring current in the power line through magneticcoupling; (ii) a sensing circuit for sensing the current in the powerline and providing a signal indicative of the sensed current; (iii) acontrol circuit receiving the signal indicative of the sensed current inthe power line and determining whether the sensed current indicates thatthe fault condition exists; and (iv) a high impedance path. The highimpedance path includes a discharging impedance circuit for limiting afault current during a fault condition. The discharging impedancecircuit injects a discharging impedance into the main power line throughmagnetic coupling during the fault condition.

In accordance with still another aspect of the present invention, thereis provided a method for limiting a fault current in a power line. Themethod includes the steps of (a) monitoring current in the power linethrough magnetic coupling; (b) sensing the current in the power line;(c) determining whether the sensed current indicates that a faultcondition exists; and (d) redirecting current to a high impedance pathif the fault condition is determined to exist, thereby limiting thefault current in the power line during the fault condition.

An advantage of the present invention is the provision of an FCL that ismore compact and lighter weight than existing FCLs.

Another advantage of the present invention is the provision of an FCLthat requires fewer components.

Still another advantage of the present invention is the provision of anFCL that is less expensive to manufacture.

Still another advantage of the present invention is the provision of anFCL that is more reliable.

Still another advantage of the present invention is the provision of anFCL that has a low normal operation loss.

Still another advantage of the present invention is the provision of anFCL that does not require service or replacement after triggering.

Yet another advantage of the present invention is the provision of anFCL that limits a fault current to a predefined level.

These and other advantages will become apparent from the followingdescription taken together with the accompanying drawings and theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take physical form in certain parts and arrangement ofparts, an embodiment of which will be described in detail in thespecification and illustrated in the accompanying drawings which form apart hereof, and wherein:

FIG. 1 is a schematic diagram showing a power system that includes anFCL according to an embodiment of the present invention;

FIG. 2A is a block diagram showing a power system that includes the FCLof the present invention in series with a secondary protection device;

FIG. 2B is a block diagram showing a power system that includes an FCLof the present invention connected in parallel with a fast-actingswitch, wherein the FCL and the switch are connected in series with asecondary protection device;

FIG. 2C is a block diagram showing a power system that includes an FCLof the present invention (without a low impedance path) connected inparallel with a fast-acting switch, wherein the FCL and the switch areconnected in series with a secondary protection device;

FIG. 3 is a detailed schematic diagram showing a 3-phase power systemthat includes an FCL according to one embodiment of the presentinvention;

FIG. 4A is an operation waveform of a pulse modulated protection scheme;

FIG. 4B is an operation waveform of a latch protection scheme;

FIG. 5 is a detailed schematic showing a sensing circuit, a switchingunit and a control circuit of one embodiment of the FCL of the presentinvention;

FIG. 6A is a schematic illustration of a 3-phase ground fault, whereinconductors of all three phases are shorted to ground simultaneously;

FIG. 6B is a schematic illustration of a 3-phase short circuit, whereinconductors of all three phases are shorted to each other;

FIG. 6C is a schematic illustration of a single line short circuit,wherein the conductor of only one phase is shorted to ground;

FIG. 6D is a schematic illustration of two line short circuit, whereinconductors of only two phases are shorted to ground, thereby causingover-current and imbalance in the system; and

FIG. 6E is a schematic illustration of a two line short circuit, whereinconductors of only two phases are shorted to each other.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein the showings are for the purposesof illustrating an embodiment of the invention only and not for thepurposes of limiting same, FIG. 1 shows a power system 2 that includes apower line 5 that electrically connects a power source 4 to a load 6.Power line 5 is comprised of one or more individual conductors. System 2also includes a fault current limiter (FCL) 40 according to anembodiment of the present invention. Power source 4 may take the form ofan AC or DC power source, including, but not limited to, a single-phaseAC generator or utility grid, a 3-phase AC generator or utility grid, ora DC generator system or power source, such as photovoltaic arrays andbatteries. In the embodiment shown in FIG. 1, power source 4 is an ACpower source.

FCL 40 is generally comprised of a magnetic coupling circuit 44; asensing circuit 60; a control circuit 70; a low impedance path 42Lincluding a reactor circuit 62, a switching unit 65, and a DC source V1(voltage or current source); and a high impedance path 42H including adischarging impedance circuit 50. Low impedance path 42L is connected inparallel with high impedance path 42H. Since power source 4 shown inFIG. 1 is an AC power source, FCL 40 also includes a rectifier 48.Rectifier 48 is included in order to convert AC current on power line 5to DC current. If power source 4 is a DC power source, then rectifier 48is omitted.

There are three principal operating zones of power system 2, namely,power line 5, current central zone (CCZ), and sensing circuit 60/controlcircuit 70. As indicated above, power line 5 connects power source 4 toload 6. Power line 5 is where a fault occurs and is the object ofprotection. The CCZ is coupled to power line 5 through magnetic couplingcircuit 44, and has a current that corresponds to the current of powerline 5.

Magnetic coupling circuit 44, which typically takes the form of anisolation transformer comprised of one or more current transformers,provides current coupling between power line 5 and the CCZ of FCL 40.Each current transformer is comprised of inductors (primary andsecondary) that are magnetically coupled (e.g., a pair of iron coremutual inductors). The primary inductor of magnetic coupling circuit 44is in series with power line 5. The secondary inductor of magneticcoupling circuit 44 is electrically connected with the CCZ. The currentin power line 5, during both normal and fault conditions, is monitoredusing magnetic coupling circuit 44, as will be explained in furtherdetail below. As indicated above, rectifier 48 is included when powersource 4 is an AC power source in order to convert AC current on powerline 5 to DC current. If power source 4 is DC, then rectifier 48 is notrequired.

Sensing circuit 60 may include any suitable current sensing device (e.g.a Hall-effect sensor or a shunt resistor). Sensing circuit 60 monitorsthe current in power line 5 and provides a signal to control circuit 70indicative of the sensed current level. It should be appreciated thatcurrent sensing can be performed at various locations, including, butnot limited, the CCZ as shown in FIG. 1, at power line 5, and at thecurrent transformers of magnetic coupling circuit 44. Accuracy may beimproved by using sensing circuit 60 to monitor the current directly atpower line 5, when inductors of magnetic coupling circuit 44 are weaklycoupled during normal conditions.

As discussed above, FCL 40 comprises low impedance path 42L and highimpedance path 42H. Low impedance path 42L includes reactor circuit 62,switching unit 65 and DC source V1. High impedance path 42H includesdischarging impedance circuit 50. During normal operation, currentconducts through low impedance path 42L, whereas during a faultcondition, current conducts through high impedance path 42H. Switchingunit 65, operated by control unit 70, switches the current flow betweenlow impedance path 42L and high impedance path 42H, as will be describedin detail below.

Reactor circuit 62 includes a DC reactor that minimizes the currentripple of the rectified DC current during normal operation. When a faultoccurs, the DC reactor provides impedance during the initial rise offault current. In this respect, the DC reactor functions to slow downthe fault current, thereby providing time for control circuit 70 toactivate switching unit 65. DC source V1 provides a DC bias duringnormal operation.

Switching unit 65 takes the form of a controllable switch, such as asemiconductor switch or a mechanical switch. Switching unit 65 has an ONstate (open switch) and an OFF state (closed switch), wherein the stateof switching unit 65 determines whether current conducts through lowimpedance path 42L or high impedance path 42H. In the illustratedembodiment, switching unit is in an ON state (closed switch) duringnormal operation and is in an OFF state (open switch) for a faultcondition. Control circuit 70 receives the signal from sensing circuit60 that is indicative of the sensed current level. When control circuit70 determines that a fault condition exists, control circuit 70activates switching unit 65 causing switching unit 65 to quickly changefrom the ON state (closed switch) to the OFF state (open switch),thereby redirecting fault current to the high impedance path 42H thatincludes discharging impedance circuit 50, which is described below.

Control circuit 70 may take various forms, including, but not limitedto, an analog control logic circuit (e.g., a comparator circuit) or aprogrammable controller (e.g., a microcontroller unit). As indicatedabove, control circuit 70 determines whether there is a fault condition.In this respect, control circuit 70 receives the signal provided bysensing circuit 60, and uses this signal to determine whether the sensedcurrent level is indicative of a fault condition. In the illustratedembodiment, control circuit 70 determines whether a fault conditionexists by determining if the current in power line 5 exceeds apredetermined threshold fault current level. If it is determined that afault condition exists, then control circuit 70 activates switching unit65 to change the state of switching unit 65, thereby causing faultcurrent to be redirected to discharging impedance circuit 50. In theillustrated embodiment, control unit 70 maintains switching unit 65 inthe ON state (closed switch) during normal operation and switchesswitching unit 65 to the OFF state (open switch) in the event of a faultcondition.

Referring now to high impedance path 42H, discharging impedance circuit50 can be resistive, reactive, or any appropriate combination. Whenfault current is redirected to discharging impedance circuit 50, faultenergy is consumed in order to limit fault current in the CCZ and onpower line 5. Discharging impedance circuit 50 may take the form ofcombinations of RLC branches so that when impedance is inserted to powerline 5 through magnetic coupling, it changes the time constant on powerline 5 and slows down the rise of fault current. This may permit themagnetic coupling to last for a longer time period and limit the faultcurrent until a secondary protection device is triggered and clears thefault.

During a fault condition, the sudden change of current in the primaryinductor of magnetic coupling circuit 44 (which is in series with powerline 5) inducts change in the magnetic field coupling the primary andsecondary inductors of magnetic coupling circuit 44. In this transient,the fault current on power line 5 is reflected to the secondary inductorof magnetic coupling circuit 44, and thus to the CCZ. As soon as thecurrent monitored by sensing circuit 60 reaches the threshold faultcurrent level, control circuit 70 outputs a trigger signal to theswitching unit 65, thereby causing switching unit to switch from the ONstate (closed switch) to the OFF state (open switch). As a result, lowimpedance path 42L is disconnected, and fault current is redirected fromlow impedance path 42L to high impedance path 42H which includesdischarging impedance circuit 50.

It is contemplated that the present invention may also be used inconjunction with a secondary protection device (e.g., fuses, circuitbreakers, protective relays, and other circuit interrupting devices).FIG. 2A is a block diagram showing a power system 2A that includes FCL40, as described above in connection with FIG. 1, and a secondaryprotection device 25. Advantageously, the size, weight and componentcosts for FCL 40 can be reduced when FCL 40 is used in conjunction witha secondary protection device that clears the fault condition within atransient time (e.g., 50 ms-200 ms). Moreover, use of FCL 40 in powersystem 2A allows the implementation of a lower-cost, slower protectiondevice 25.

When FCL 40 is used in conjunction with secondary protection device 25,FCL 40 serves as a buffer for secondary protection device 25. In thisrespect, FCL 40 is active for a very short period of time (e.g., about50 ms-200 ms) after the occurrence of a fault condition. Thereafter,secondary protection device 25 is activated to interrupt the circuitbetween power source 4 and the location of the fault on power line 5,thereby totally removing the fault current. In this embodiment, the highimpedance path 42H limits the fault current for a period of time untilprotection device 25 clears the fault condition.

The use of FCL 40 in combination with secondary protection device 25allows the size, cost and complexity of secondary protection device 25to be reduced. Since FCL 40 limits the fault current, a lower currentrated secondary protection device 25 is used. For example, high currentrated circuit breakers are bulky and expensive. These can be eliminatedwith proper FCL operation, where FCL 40 holds the current below athreshold current for a time period long enough for the low currentrated secondary protection device 25 to clear the fault.

FIG. 2B illustrates a power system 2B in which FCL 40 is used incombination with a secondary protection device 25 (e.g., a conventionalfuse or circuit breaker), wherein FCL 40 is arranged in parallel with aswitch 23 moveable between an ON (closed) position and an OFF (open)position. Secondary protection device 25 is connected in series with theparallel-connected FCL 40 and switch 23. Switch 23 is preferably afast-acting switch having a relatively small current rating. In thisregard, switch 23 may be a fuse, a circuit breaker, a semiconductorswitch, or any other type of fast response protection device that opensquickly. It is noted that fuses have very low impedance when conductingin the normal state, and therefore the power loss of such device isalmost negligible in the normal state.

During normal operation, both switch 23 and secondary protection device25 are closed, thereby conducting normal current. When a fault occursdownstream and the current on power line 5 reaches the predeterminedthreshold fault current level, switch 23 quickly opens (i.e., turns OFF)to redirect the fault current to FCL 40. FCL 40 limits the fault currentuntil secondary protection device 25 disconnects power source 4 fromload 6 within a short time period (e.g., 100-200 ms), thereby clearingthe fault. In this embodiment, high impedance path 42H is magneticallycoupled across switch 23. When switch 23 opens, the high impedance isautomatically inserted into power line 5 line through magnetic couplingcircuit 44. Since current flows through FCL 40 only during a faultcondition, FCL 40 does not need to include low impedance path 42L. Lowimpedance path 42L may be effectively removed from FCL 40 by keepingswitching unit 65 in the OFF state (open) during both normal and faultconditions.

Alternatively, low impedance path 42L may be removed by omitting thecomponents of low impedance path 42L. In this regard, FIG. 2Cillustrates a power system 2C having an FCL 40′ that omits sensingcircuit 60, reactor circuit 62, switching unit 65, DC source V1 andcontrol circuit 70. In the event of a fault, when the fault currentreaches the predetermined threshold fault current level, switch 23 opens(i.e., turns OFF) within a very short period of time. As a result, thefault current is redirected to FCL 40′. FCL 40′ clamps the fault currentusing discharging impedance circuit 50 of high impedance path 42H.Secondary protection device 25 disconnects power source 4 from load 6within a short time period.

Referring now to power system 2B (FIG. 2B) and power system 2C (FIG.2C), when switch 23 opens (turns OFF), there is a small arc acrossswitch 23, since the energy is being redirected to FCL 40, 40′. In thiscase, the ratings for switch 23 can be low. Moreover, since secondaryprotection device 25 interrupts the current at the current level that islimited by FCL 40, 40′, the required rating of secondary protectiondevice 25 is also low. As a result, switch 23 and secondary protectiondevice 25 can be low cost and have small volume and size.

FIG. 3 shows a three-phase power system 30 that includes a three-phase(AC power) power line 36 (comprised of conductors 36 a, 36 b and 36 c)which electrically connects a three-phase voltage source 32 to athree-phase load 38. Three-phase voltage source 32 has a sourceimpedance 34 represented by inductors Ls and resistors rs on each of theconductors 36 a, 36 b, 36 c. System 30 also includes fault currentlimiter (FCL) 40 according to an embodiment of the present invention.

In the embodiment illustrated in FIG. 3, FCL 40 is generally comprisedof a magnetic coupling circuit 44 that takes the form of an isolationtransformer comprised of three (3) single-phase current transformers 45(or alternatively a single three-phase current transformer); a rectifier48; a sensing circuit 60; a low impedance path 42L including a reactorcircuit 62, a switching unit 65, and a DC source V1 (voltage or currentsource); a high impedance path 4211 including a discharging impedancecircuit 50; and a control circuit 70. Low impedance path 42L isconnected in parallel with high impedance path 42H.

Magnetic coupling circuit 44 is used to monitor the current in powerline 36. In the illustrated embodiment, the AC power line current ismirrored through the three single-phase current transformers 45 (eachhaving a turns ratio of 1:1) to the secondary sides of the currenttransformers 45 that are electrically connected to rectifier 48.

Rectifier 48 rectifies the AC current mirrored from current transformers45 into DC current. In the illustrated embodiment, rectifier 48 iscomprised of a full-bridge rectifier that includes six rectifier diodesD1. The AC power line current is monitored through magnetic coupling,which is non-intrusive to conductors 36 a, 36 b, 36 c that are beingprotected.

Sensing circuit 60 senses a DC current and provides an output Vin thatis proportional to the sensed current Idc. As indicated above, sensingcircuit 60 may take the form of a Hall-effect sensor or a shuntresistor.

In the illustrated embodiment, reactor circuit 62 of low impedance path42L is comprised of a DC reactor in the form of a reactor inductor coilLd, a resistor rd, and a freewheeling diode D3. Reactor inductor coil Ldis series-connected with resistor rd. Diode D3 is connected in parallelwith reactor inductor coil Ld and resistor rd.

In the embodiment shown in FIG. 3, switching unit 65 of low impedancepath 42L takes the form of a semiconductor switch, such as an insulatedgate bipolar transistor (IGBT), a MOSFET, or a HET, with an associateddriving circuit. In the illustrated embodiment, the semiconductor switchis an IGBT (e.g., Advanced Power Technology APT50GF100BN). However, aMOSFET improves the power loss on switching unit 65 during normaloperation. It is contemplated that other types of switches may be used,including other semiconductor switches, as well as mechanical switches.

DC source V1 of low impedance path 42L provides the necessary DC biasduring normal operation. The value of DC source V1 is selected such thatthe normal operation current is maintained.

In the illustrated embodiment, discharging impedance circuit 50 of highimpedance path 42H is comprised of a discharging resistor 52 (having aresistance value rp) series-connected with a blocking diode D2 forblocking leakage current. It is also contemplated that dischargingresistor 52 may be part of an RLC circuit.

Control circuit 70 as shown in FIG. 3 includes a comparator 76 (e.g.,Texas Instruments LM393 voltage comparator) and an associated latchcircuit that includes feedback diode D4. Vin from sensing circuit 60reflects the sensed current level (I_(dc)). Comparator 76 compares Vinwith Vref which is a predetermined constant value that represents athreshold fault current level. When Vin<Vref, then Vout is in a lowstate (indicating a normal condition). As a result, switching unit 65 isin the ON state (i.e., closed switch). When Vin>Vref, then the output(Vout) of comparator 76 flips from the low state to a high state(indicating a fault condition), and feedback diode D4 conducts, pullingVin to a high state. In this case, Vin remains larger than Vref and Voutis “latched” at the high state. Therefore, when Vin>Vref, the resultingVout causes switching unit 65 to change from the ON state (normaloperation) to the OFF state (current limiting), and remain in the OFFstate once it is triggered. Switching unit 65 maintains the OFF stateregardless of changes to the sensed current level (I_(dc)), untilcontrol circuit 70 is reset. Upon reset, switching unit 65 is returnedto the ON state. This latching scheme helps reduce voltage stress acrossthe semiconductor switch of switching unit 65.

It should be appreciated that while the embodiment illustrated by FIG. 3shows control circuit 70 comprised of an analog control logic circuit(e.g., a comparator circuit), it is also contemplated that controlcircuit 70 may take the form of a programmable controller (e.g., amicrocontroller unit).

During normal operation, high impedance path 42H is bypassed due to theON state (closed switch) of switching unit 65. In contrast, during afault condition switching unit 65 switches from the ON state (closedswitch) to the OFF state (open switch) to replace low impedance path 42Lwith high impedance path 42H, and thereby force fault current to passthrough high impedance path 42H. Accordingly, the fault current islimited by discharging impedance circuit 50 (i.e., discharging resistor52 and blocking diode D2). Discharging resistor 52 dissipates virtuallyno power unless there is a fault current. The resistance value rp ofdischarging resistor 52 is selected such that the fault current isclamped at a desire level upon triggering of the switching unit 65 tochange switching unit 65 from the ON state (closed switch) to the OFFstate (open switch).

When switching unit 65 in low inductance path 42L is switched to the OFFstate, current still remains in the DC reactor of reactor circuit 62.Freewheeling diode D3 (which is connected in parallel with the DCreactor) allows this residual current in the DC reactor to continue toflow within a small loop.

As will be explained below, selection of the resistance value rp fordischarging resistor 52 leads to two different protection schemes: (i)pulse modulated protection and (ii) latch protection. Pulse modulatedprotection scheme is described in detail in Hagh et al. The pulsemodulated protection scheme provides more flexibility in adjusting thethreshold fault current level (i.e., the current level at which FCL 40is activated to limit the current) with a fixed resistance value rp. Inthe illustrated embodiment, the threshold fault current level may becustomized by the user.

Referring now to FIG. 4A, there is shown an operation waveform of apulse modulated protection scheme, wherein

I_(threshold) is the threshold fault current level that triggers theIGBT OFF,

I_(normal) is the current level during normal operation,

I_(dc) is the DC current sensed by sensing circuit 60,

V_(igbt) is the voltage across the semiconductor switch (IGBT), and

I_(igbt) is the current through the semiconductor switch (IGBT).

With a fixed discharging resistor value rp, the pulse modulatedprotection scheme provides a wide range of adjustable fault currentlimiting levels by varying the duty ratio of the pulse signal. However,one limitation of the pulse modulated protection scheme is that thevoltage stress on the semiconductor switch can get very high dependingon the duty ratio. For example, in a three-phase system with aline-to-line voltage (i.e., the voltage measured between conductors 36a, 36 b and 36 c) of 600V, the peak voltage V_(igbt) across thesemiconductor switch would be as high as 4 kV if the fault current islimited at 30 A.

Referring now to FIG. 4B, there is shown an operation waveform of alatch protection scheme. In the latch protection scheme “latching”occurs after a one-shot action of the semiconductor switch in order tolower the voltage stress of the semiconductor switch. In the latchprotection scheme, the discharging resistor value rp is fine tuned suchthat the fault current is held at a desired level when it is conductedto pass rp continuously.

The latch protection can be modified to a pulse modulated scheme byadding pulse width modulation (PWM) control. In this case, the range ofadjustable fault current limiting level is reduced by a smallerdischarging resistor value rp.

The present invention will now be further described by way of thefollowing example FCL 40.

EXAMPLE

Example FCL 40 will now be described with reference to FIG. 5 andTABLE 1. TABLE 1 provides a list of major components selected for thesample prototype.

TABLE 1 Device Part Description current CR MAGNETICS 1:1 ratio, 40 A,1.5 lbs. transformers 10WP-005 45 rectifier DIODES Inc. 1000 V reversevoltage diodes D1 10A07-T for rectifier 48 switching FAIRCHILD 600 V, 20A 3-phase IGBT unit 65 FSAM20SH60A inverter bridge with drivers, 40 AIGBT collector current (peak) discharging Yageo 80 W, 15 Ω resistor 52AHA80AJB-15R-ND blocking DIODES Inc. 1.5 A surface mount glass diode D2S2AA passivated rectifier reactor MURATA 680 μH, 3.1 A, quantity = 6inductor 1400 Series coil Ld 1468431C sensing Honeywell miniatureratiometric linear Hall- circuit 60 CSLH3A45 effect sensor

Current transformers 45 have a low stress rating during normal operation(30V, 5 A and 210 W), and are highly stressed during a fault condition(450V, 30 A and 14 kW). Since the fault condition typically lasts foronly about 200 ms, current transformers 45 can be reasonably undersizedas long as they can handle the peak power in 200 ms or handle the peakpower until a current secondary protection device clears the fault. Itshould be noted that the current transformers 45 identified in TABLE 1are designed for metering purposes, and therefore weight and cost can befurther reduced if transformers 45 are custom made with less accuracy.

Referring to FIG. 5, there is shown a detailed schematic showing sensingcircuit 60, switching unit 65 and control circuit 70 for the example FCL40. It should be appreciated that the circuit design of FIG. 5 is shownfor the purpose of illustrating an embodiment of the present invention,and is not intended to limit same.

The selected switching unit 65 includes a semiconductor switch 66 thattakes the form of an IGBT. However, it is contemplated that a MOSFET,JFET, or other suitable switching device can be used instead of an IGBTin order to improve the power loss on switching unit 65 during normaloperation. With a MOSFET, impedance is ˜0.1 Ohms if two MOSFETs areparallel connected, which is 2.5 W power dissipation when normaloperation current is 5 A. The IGBT identified in TABLE 1 normally has acollector-emitter saturation voltage of 2.5V, which dissipates 12.5 Wwhen conducting the 5 A current. Switching unit 65 should be energizedbefore FCL 40 is connected to power line 36, since switching unit 65conducts continuous operation current during normal operation.

Sensing circuit 60 is shown as a hall-effect current sensing device.Comparator 76 of control circuit 70 may take the form of a TexasInstruments LM393 voltage comparator. It should be appreciated thatalternative sensing devices and voltage comparators may be substitutedfor the illustrated components.

A power supply 80 provides power to sensing circuit 60. In theillustrated embodiment, power supply 80 is comprised of a 9V battery 82,regulated to 5V by a voltage regulator 84. Divided by a potentiometer 78of 10K, the 9V-battery 82 (regulated to 5V) also provides a referenceinput voltage (Vref) to the inverse input terminal of comparator 76 ofcontrol circuit 70. The DC power provided by the 9V-battery couldalternatively be provided by a DC/DC converter, an AC/DC converter orother power supply device.

Switching unit 65 is powered by a power supply 90. In the illustratedembodiment, power supply 90 is a 15V DC power supply. The driver for theIGBT requires a low signal to turn ON the IGBT (closed switch) and ahigh signal to turn OFF the IGBT (open switch). Feedback diode D4 isprovided to carry out latching, as described above.

The fault types identified in a 3-phase system are summarized withreference to FIGS. 6A-6E. There are five different types of typicalfaults in 3-phase systems. These faults are categorized into twodifferent groups: (i) symmetric faults (3-phase ground fault and 3-phaseshort circuit) and (ii) asymmetric faults (single phase ground fault,2-phase ground fault, and 2-phase short circuit). The most common faultsare line-to-ground faults. Line-to-line faults account for about 15% ofall short-circuit faults, while symmetric 3-phase faults only accountfor about 5% of all short circuit faults.

FIG. 6A schematically illustrates a 3-phase ground fault, wherein theconductors of all three phases are shorted to ground simultaneously. Thefault impedances on all three phases are identical, maintaining thebalance among the three phases. During this type of fault, the loadimpedance drops to virtually zero, causing large fault currents on allthree phases running into ground. FIG. 6B schematically illustrates a3-phase short circuit, wherein the conductors of all three phases areshorted to each other. FIG. 6C schematically illustrates a single lineshorted to ground, wherein a conductor of only one phase is shorted toground. Simulations show that in case of this type of fault, the faultcurrent would not be as large as in the 3-phase faults mentioned above,FIG. 6D schematically illustrates two lines shorted to ground, whereinconductors of only two phases are shorted to ground, thereby causingover-current and unbalance in the system. FIG. 6E schematicallyillustrates a two line short circuit, wherein conductors of only twophases are shorted to each other. Simulations show that the behavior andresponse of FCL 40 in the case of a 2-phase short circuit (shorted toeach other) is virtually identical to those in the case of a 2-phaseground fault.

The present invention provides a FCL 40 that limits the fault current ina power system 30 during a fault condition. When the fault currentexceeds a predetermined threshold fault current level, FCL 40 istriggered thereby limiting the fault current under a desired level. FCL40 limits the fault current during a fault condition, while insertingonly a negligible impedance to power line 36 during normal conditions.In one embodiment of the present invention, FCL 40 uses a magneticcoupling circuit 44 to couple the AC current on power line 36 to thesecondary side, and rectifies this AC current to a DC current that islet through by a normally-closed semiconductor switch 66 during normalconditions. Since the AC and DC currents are coupled with each other,the primary side AC current can therefore be controlled by controllingthe DC current on the secondary side. In the event of a short circuitfault on the power line and the primary side AC current reaching thepredetermined threshold fault current level, an impedance is insertedinto the DC side to control the DC current, and consequently, to limitthe AC current to the desired level. This process occurs quickly (i.e.,microseconds), thereby preventing the presence of a large overshootcurrent that is potentially hazardous to the secondary protectiondevices and downstream equipment.

Unlike existing FCLs, FCL 40 uses magnetic coupling to monitor andcontrol an AC current on the primary side of the power line. Thisnon-intrusive approach advantageously has small power dissipation (andequivalently, the inserted impedance) during normal operation. Moreover,during a fault condition, a large impedance is inserted in the powerline (through magnetic coupling) to limit fault current quickly (i.e.,in microseconds), thereby preventing a large overshoot of fault currentthat can damage other equipment in the power system.

It should be appreciated that several modifications to FCL 40 arecontemplated. For example, size and weight can be further reduced forcommercialized products. Furthermore, as noted above, two differentschemes of current limiting may be implemented in accordance with thepresent invention. These schemes are (i) pulse width modulatedprotection, which provides a self-recoverable feature of FCL 40 and awider range of adjusting for the threshold fault current level, and (ii)latch protection, which only requires a “reset” action after everytriggering, while it effectively lowers the voltage stress of thesemiconductor switch, thereby lowering the cost and size of FCL 40. Itis contemplated that FCL 40 can be either self-resettable or can bemanually reset by pushing a button or via remote control.

As indicated above, it is contemplated that the FCL of the presentinvention may be used in combination with secondary protection devices(e.g., conventional circuit breaker, protection relays, and/or fuses) toclear a fault. When used with secondary protection devices, the FCL ofthe present invention reacts faster than the secondary protectiondevices trigger, thereby permitting the use of secondary protectiondevices having lower response speed and current rating.

Conventional secondary protection devices typically have delays inprotecting against over-current conditions. FCL 40, for example, canrespond to the fault current fast, and limit the fault current for thesecondary protection devices to prevent damage. In turn, the secondaryprotection devices can clear the fault current and terminate currentflow on the power line in a short time, thereby reducing the high energystress on components of the FCL. Since the fault clearing time is veryshort, the size and cost of FCL 40 can be reduced because currenttransformers 45 and discharging resistor 52 can be smaller.

As indicated above, the present invention provides several advantageswith respect to compactness, reduced weight, simplicity, reliability andlow normal operation loss. In this respect, since the components arerated for limiting a large fault current for a few hundred milliseconds,the components can be small and lightweight, as compared to componentsrated to run the fault current in steady state. Furthermore, most of thecomponents of the FCL of the present invention are passive elements, andthus control elements only need to be activated when the fault currentreaches the threshold fault current level. Both the configuration andoperation of the FCL are simple and therefore the FCL provides enhancedreliability.

Since switching unit 65 of FCL 40 only switches when there is a faultcurrent, there is no switching loss during normal operation. The totallosses in normal condition are the voltage drop of solid state elements,and the losses within current transformers 45. Since the efficiency ofcurrent transformers 45 can be very high, and the voltage drop onsemiconductor switch 66 is low, the total loss during normal operationof FCL 40 is low. In addition, the FCL 40 does not require action ormaintenance after triggered by a fault. As long as the AC line currenton the primary side of current transformers 45 is below the thresholdfault current level, semiconductor switch 66 is set to the ON state(closed switch) and FCL 40 is operating under normal operation mode. Theprotection process could be self-recoverable and repeatable withouthuman service.

In the FCL of the present invention, the fault current may be allowed toincrease quickly, and in some cases temporarily above the clampedthreshold current level, before switching unit 65 is changed to the OFFstate (open switch). In this respect, switch action in IGBTs and MOSFETsare so fast (often on the order of microseconds), that it is not alwaysnecessary to clamp the peak fault current in the first short transienttime periods. Since the DC reactor of reactor circuit 62 does not needto limit the rate of faulted current change and does not need to carrythe faulted current, its size, weight and inductance value can besubstantially reduced.

It is further contemplated that the FCL of the present invention may bemodified to remove DC reactor circuit 62 from low impedance path 42L ifcontrol circuit 70 can change switching unit 65 to the OFF state (openswitch) quickly enough and the current ripple is not too large.

The foregoing description is a specific embodiment of the presentinvention. It should be appreciated that this embodiment is describedfor purposes of illustration only, and that numerous alterations andmodifications may be practiced by those skilled in the art withoutdeparting from the spirit and scope of the invention. It is intendedthat all such modifications and alterations be included insofar as theycome within the scope of the invention as claimed or the equivalentsthereof.

Having described the invention, the following is claimed:
 1. A faultcurrent limiter (FCL) for limiting a fault current in a power lineduring a fault condition, said FCL comprising: a magnetic couplingcircuit for monitoring current in the power line through magneticcoupling; a sensing circuit for sensing the current in the power lineand providing a signal indicative of the sensed current; a controlcircuit receiving the signal indicative of the sensed current in thepower line and determining whether the sensed current indicates that thefault condition exists; a high impedance path including: a dischargingimpedance circuit for limiting the fault current, said dischargingimpedance circuit injecting a discharging impedance into the main powerline through magnetic coupling during the fault condition; and a lowimpedance path connected in parallel with the high impedance path, saidlow impedance path including: a reactor circuit; and a switching unithaving an ON state and an OFF state, wherein the state of the switchingunit determines whether current conducts through the low impedance pathor the high impedance path, said control circuit determining the stateof the switching unit.
 2. A FCL according to claim 1, wherein saidcontrol circuit determines whether the sensed current indicates that thefault condition exists by determining if the sensed current exceeds athreshold fault current level indicative of the fault condition.
 3. AFCL according to claim 1, wherein said control unit switches theswitching unit from the ON state to the OFF state in response todetermining that the fault condition exists, thereby redirecting saidfault current from the low impedance path to the high impedance path. 4.A FCL according to claim 1, wherein said reactor circuit includes a DCreactor.
 5. A FCL according to claim 4, wherein said reactor circuitfurther includes a freewheeling diode in parallel with said DC reactor.6. A FCL according to claim 1, wherein said reactor circuit is connectedin series with said switching unit.
 7. A FCL according to claim 6,wherein said low impedance path further includes a DC source connectedin series with the reactor circuit and the switching unit, to provide aDC bias during normal operation.
 8. A FCL according to claim 1, whereinsaid discharging impedance circuit includes a discharging resistor or anRLC circuit.
 9. A FCL according to claim 8, wherein said dischargingimpedance circuit further includes a blocking diode connected in serieswith said discharging resistor.
 10. A FCL according to claim 1, whereinsaid switching unit comprises a semiconductor switch or a mechanicalswitch.
 11. A FCL according to claim 10, wherein said semiconductorswitch is one of the following: an IGBT, a MOSFET or a JFET.
 12. A FCLaccording to claim 1, wherein said control circuit includes aprogrammable controller.
 13. A FCL according to claim 1, wherein saidcontrol circuit includes a comparator.
 14. A FCL according to claim 13,wherein said control circuit further includes a feedback diode.
 15. AFCL according to claim 1, wherein said power line is connected with a DCpower source.
 16. A FCL according to claim 1, wherein said power line isconnected with an AC power source.
 17. A FCL according to claim 1,wherein said magnetic coupling circuit comprises at least one currenttransformer.
 18. A FCL according to claim 1, wherein said sensingcircuit includes one of the following: a hall-effect current sensingdevice or a shunt resistor.
 19. A power system comprising: a power linethat electrically connects a power source to a toad; a protection devicefor interrupting a circuit to clear a fault condition; and a faultcurrent limiter (FCL) for limiting a fault current in the power lineduring a fault condition, said FCL comprising: a magnetic couplingcircuit for monitoring current in the power line through magneticcoupling; a sensing circuit for sensing the current in the power lineand providing a signal indicative of the sensed current; a controlcircuit receiving the signal indicative of the sensed current in thepower line and determining whether the sensed current indicates that thefault condition exists; and a high impedance path including: adischarging impedance circuit for limiting a fault current during afault condition, said discharging impedance circuit injecting adischarging impedance into the main power line through magnetic couplingduring the fault condition, a low impedance path in parallel with thehigh impedance path, said low impedance path including: a reactorcircuit; and a switching unit having an ON state and an OFF state,wherein the state of the switching unit determines whether currentconducts through the low impedance path or the high impedance path, saidcontrol circuit determining the state of the switching unit.
 20. A powersystem according to claim 19, wherein said protection device includesone of the following: a fuse, a circuit breaker, or a protective relay.21. A power system according to claim 19, wherein said high impedancepath limits the fault current for a period of time until said protectiondevice clears the fault condition.
 22. A power system according to claim19, wherein said control circuit determines whether the sensed currentindicates that the fault condition exists by determining if the sensedcurrent exceeds a threshold fault current level indicative of the faultcondition.
 23. A power system according to claim 19, wherein said FCLfurther comprises: a rectifier for converting AC current on the powerline to DC current.
 24. A method for limiting a fault current in a powerline, comprising: monitoring current in the power line through magneticcoupling; sensing the current in the power line; determining whether thesensed current indicates that a fault condition exists; snf if the faultcondition is determined to exist, then redirecting current from a lowimpedance path to a high impedance path that is connected in parallelwith the low impedance path, if the fault condition is determined toexist, thereby limiting the fault current in the power line during thefault condition, wherein the high impedance path includes a dischargingimpedance circuit for limiting the fault current, said dischargingimpedance circuit injecting a discharging impedance into the main powerline through magnetic coupling during the fault condition, and the lowimpedance path includes a reactor circuit and a switching unit having anON state and an OFF state, wherein the state of the switching unitdetermines whether current conducts through the low impedance path orthe high impedance path, said control circuit determining the state ofthe switching unit.
 25. A method according to claim 24, wherein saidstep of determining whether a fault condition exists includes:determining whether the current sensed in the power line exceeds apredetermined threshold fault current level.
 26. A method according toclaim 24, wherein said method further comprises: activating a switchingunit in response to determining that the fault condition exists, whereinactivation of the switching unit redirects the current to the highimpedance path.